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The fragile X syndrome is the most common inherited form of mental retardation. In addition to moderate to severe mental retardation, affected males have facial dysmorphic features, macroorchidism, and a folate-sensitive fragile site on the X chromosome, at band Xq27.3. The female heterozygotes may also exhibit mental retardation. The responsible gene (FMR-1) has been cloned, and it includes the repeated CGG (cytosine-guanine-guanine) sequence that causes the fragile X mutation. Normal individuals have 2-50 CGG repeats at this locus, with an average of about 30 repeats. Alleles with more than about 50 repeats, including those identified in a normal family, are meiotically unstable. Individuals with no significant phenotypic abnormalities, but who are at high risk for having an affected offspring, have triple repeats ranging between 50 and 200. Such individuals are said to have a premutation, because it has a high probability of expanding to a full mutation in the next generation. Individuals with more that 200 CGG repeats are affected with the fragile X syndrome. The passage from premutation to full mutation status occurs only with transmission from the mother. Expanded CGG repeats appear to cause abnormal methylation of the FMR-1 gene, which, in turn, causes a reduction in gene transcription, resulting in disease. Molecular testing for clinical diagnosis, cartier detection, and prenatal diagnosis of the fragile X syndrome is available and is proving more reliable than cytogenetic testing. Results from chorionic villus sampling should be interpreted with caution because the methylation status of the FMR-1 gene is rarely established in chorionic villi at the time of sampling. Chorionic villus sampling, although a standard technique for prenatal diagnosis, may lead to a situation where follow-up amniocentesis is necessary to resolve an ambiguous result.
Kennedy’s Disease
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Kennedy’s disease (spinal and bulbar muscular atrophy) is an X-linked recessive disorder characterized by motor neuron degeneration that starts in adulthood and leads to progressive muscular weakness of the upper and lower extremities and late onset of mild androgen insensitivity. Affected males have reduced fertility and gynecomastia; female carriers have few or no symptoms. A mutation in the N terminus of the androgen receptor gene involving CAG repeats has been shown to be the cause of this disease. Unaffected individuals have 13-30 CAG repeats in the first exon (ie, transcribed region) of this gene; individuals with Kennedy’s disease have 40-62 repeats.
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